Postmark is an email delivery service for developers, made by ActiveCampaign. A program hands it an email through a REST API (an interface called over HTTPS) or through SMTP, the standard protocol for sending mail, and Postmark delivers the message to the recipient's inbox. Official libraries exist for Node.js, Ruby and Rails, PHP, .NET, Python and other languages.
Postmark separates transactional email, messages triggered by a user action such as a password reset or a notification, from promotional email such as newsletters. Its Message Streams send the two kinds through separate sending infrastructure, so bulk mail does not slow down or harm the delivery of transactional mail. Postmark says it vets every new sender and never lets spammers use the service. Other features are email templates, event webhooks, analytics on delivery, bounces, spam complaints and opens, a Bulk API for batch sending, and inbound email processing, which accepts email sent to the application.
Postmark is a hosted service with monthly plans priced by email volume. A free Developer plan includes 100 emails a month and does not expire.